async.c

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/*-------------------------------------------------------------------------
 *
 * async.c
 *    Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/commands/async.c
 *
 *-------------------------------------------------------------------------
 */
 
/*-------------------------------------------------------------------------
 * Async Notification Model as of 9.0:
 *
 * 1. Multiple backends on same machine. Multiple backends listening on
 *    several channels. (Channels are also called "conditions" in other
 *    parts of the code.)
 *
 * 2. There is one central queue in disk-based storage (directory pg_notify/),
 *    with actively-used pages mapped into shared memory by the slru.c module.
 *    All notification messages are placed in the queue and later read out
 *    by listening backends.
 *
 *    There is no central knowledge of which backend listens on which channel;
 *    every backend has its own list of interesting channels.
 *
 *    Although there is only one queue, notifications are treated as being
 *    database-local; this is done by including the sender's database OID
 *    in each notification message.  Listening backends ignore messages
 *    that don't match their database OID.  This is important because it
 *    ensures senders and receivers have the same database encoding and won't
 *    misinterpret non-ASCII text in the channel name or payload string.
 *
 *    Since notifications are not expected to survive database crashes,
 *    we can simply clean out the pg_notify data at any reboot, and there
 *    is no need for WAL support or fsync'ing.
 *
 * 3. Every backend that is listening on at least one channel registers by
 *    entering its PID into the array in AsyncQueueControl. It then scans all
 *    incoming notifications in the central queue and first compares the
 *    database OID of the notification with its own database OID and then
 *    compares the notified channel with the list of channels that it listens
 *    to. In case there is a match it delivers the notification event to its
 *    frontend.  Non-matching events are simply skipped.
 *
 * 4. The NOTIFY statement (routine Async_Notify) stores the notification in
 *    a backend-local list which will not be processed until transaction end.
 *
 *    Duplicate notifications from the same transaction are sent out as one
 *    notification only. This is done to save work when for example a trigger
 *    on a 2 million row table fires a notification for each row that has been
 *    changed. If the application needs to receive every single notification
 *    that has been sent, it can easily add some unique string into the extra
 *    payload parameter.
 *
 *    When the transaction is ready to commit, PreCommit_Notify() adds the
 *    pending notifications to the head of the queue. The head pointer of the
 *    queue always points to the next free position and a position is just a
 *    page number and the offset in that page. This is done before marking the
 *    transaction as committed in clog. If we run into problems writing the
 *    notifications, we can still call elog(ERROR, ...) and the transaction
 *    will roll back.
 *
 *    Once we have put all of the notifications into the queue, we return to
 *    CommitTransaction() which will then do the actual transaction commit.
 *
 *    After commit we are called another time (AtCommit_Notify()). Here we
 *    make any actual updates to the effective listen state (listenChannels).
 *    Then we signal any backends that may be interested in our messages
 *    (including our own backend, if listening).  This is done by
 *    SignalBackends(), which scans the list of listening backends and sends a
 *    PROCSIG_NOTIFY_INTERRUPT signal to every listening backend (we don't
 *    know which backend is listening on which channel so we must signal them
 *    all).  We can exclude backends that are already up to date, though, and
 *    we can also exclude backends that are in other databases (unless they
 *    are way behind and should be kicked to make them advance their
 *    pointers).
 *
 *    Finally, after we are out of the transaction altogether and about to go
 *    idle, we scan the queue for messages that need to be sent to our
 *    frontend (which might be notifies from other backends, or self-notifies
 *    from our own).  This step is not part of the CommitTransaction sequence
 *    for two important reasons.  First, we could get errors while sending
 *    data to our frontend, and it's really bad for errors to happen in
 *    post-commit cleanup.  Second, in cases where a procedure issues commits
 *    within a single frontend command, we don't want to send notifies to our
 *    frontend until the command is done; but notifies to other backends
 *    should go out immediately after each commit.
 *
 * 5. Upon receipt of a PROCSIG_NOTIFY_INTERRUPT signal, the signal handler
 *    sets the process's latch, which triggers the event to be processed
 *    immediately if this backend is idle (i.e., it is waiting for a frontend
 *    command and is not within a transaction block. C.f.
 *    ProcessClientReadInterrupt()).  Otherwise the handler may only set a
 *    flag, which will cause the processing to occur just before we next go
 *    idle.
 *
 *    Inbound-notify processing consists of reading all of the notifications
 *    that have arrived since scanning last time. We read every notification
 *    until we reach either a notification from an uncommitted transaction or
 *    the head pointer's position.
 *
 * 6. To avoid SLRU wraparound and limit disk space consumption, the tail
 *    pointer needs to be advanced so that old pages can be truncated.
 *    This is relatively expensive (notably, it requires an exclusive lock),
 *    so we don't want to do it often.  We make sending backends do this work
 *    if they advanced the queue head into a new page, but only once every
 *    QUEUE_CLEANUP_DELAY pages.
 *
 * An application that listens on the same channel it notifies will get
 * NOTIFY messages for its own NOTIFYs.  These can be ignored, if not useful,
 * by comparing be_pid in the NOTIFY message to the application's own backend's
 * PID.  (As of FE/BE protocol 2.0, the backend's PID is provided to the
 * frontend during startup.)  The above design guarantees that notifies from
 * other backends will never be missed by ignoring self-notifies.
 *
 * The amount of shared memory used for notify management (NUM_NOTIFY_BUFFERS)
 * can be varied without affecting anything but performance.  The maximum
 * amount of notification data that can be queued at one time is determined
 * by slru.c's wraparound limit; see QUEUE_MAX_PAGE below.
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <limits.h>
#include <unistd.h>
#include <signal.h>
 
#include "access/parallel.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/pg_database.h"
#include "commands/async.h"
#include "common/hashfn.h"
#include "funcapi.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/sinval.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
 
 
/*
 * Maximum size of a NOTIFY payload, including terminating NULL.  This
 * must be kept small enough so that a notification message fits on one
 * SLRU page.  The magic fudge factor here is noncritical as long as it's
 * more than AsyncQueueEntryEmptySize --- we make it significantly bigger
 * than that, so changes in that data structure won't affect user-visible
 * restrictions.
 */
#define NOTIFY_PAYLOAD_MAX_LENGTH   (BLCKSZ - NAMEDATALEN - 128)
 
/*
 * Struct representing an entry in the global notify queue
 *
 * This struct declaration has the maximal length, but in a real queue entry
 * the data area is only big enough for the actual channel and payload strings
 * (each null-terminated).  AsyncQueueEntryEmptySize is the minimum possible
 * entry size, if both channel and payload strings are empty (but note it
 * doesn't include alignment padding).
 *
 * The "length" field should always be rounded up to the next QUEUEALIGN
 * multiple so that all fields are properly aligned.
 */
typedef struct AsyncQueueEntry
{
    int         length;         /* total allocated length of entry */
    Oid         dboid;          /* sender's database OID */
    TransactionId xid;          /* sender's XID */
    int32       srcPid;         /* sender's PID */
    char        data[NAMEDATALEN + NOTIFY_PAYLOAD_MAX_LENGTH];
} AsyncQueueEntry;
 
/* Currently, no field of AsyncQueueEntry requires more than int alignment */
#define QUEUEALIGN(len)     INTALIGN(len)
 
#define AsyncQueueEntryEmptySize    (offsetof(AsyncQueueEntry, data) + 2)
 
/*
 * Struct describing a queue position, and assorted macros for working with it
 */
typedef struct QueuePosition
{
    int         page;           /* SLRU page number */
    int         offset;         /* byte offset within page */
} QueuePosition;
 
#define QUEUE_POS_PAGE(x)       ((x).page)
#define QUEUE_POS_OFFSET(x)     ((x).offset)
 
#define SET_QUEUE_POS(x,y,z) \
    do { \
        (x).page = (y); \
        (x).offset = (z); \
    } while (0)
 
#define QUEUE_POS_EQUAL(x,y) \
    ((x).page == (y).page && (x).offset == (y).offset)
 
#define QUEUE_POS_IS_ZERO(x) \
    ((x).page == 0 && (x).offset == 0)
 
/* choose logically smaller QueuePosition */
#define QUEUE_POS_MIN(x,y) \
    (asyncQueuePagePrecedes((x).page, (y).page) ? (x) : \
     (x).page != (y).page ? (y) : \
     (x).offset < (y).offset ? (x) : (y))
 
/* choose logically larger QueuePosition */
#define QUEUE_POS_MAX(x,y) \
    (asyncQueuePagePrecedes((x).page, (y).page) ? (y) : \
     (x).page != (y).page ? (x) : \
     (x).offset > (y).offset ? (x) : (y))
 
/*
 * Parameter determining how often we try to advance the tail pointer:
 * we do that after every QUEUE_CLEANUP_DELAY pages of NOTIFY data.  This is
 * also the distance by which a backend in another database needs to be
 * behind before we'll decide we need to wake it up to advance its pointer.
 *
 * Resist the temptation to make this really large.  While that would save
 * work in some places, it would add cost in others.  In particular, this
 * should likely be less than NUM_NOTIFY_BUFFERS, to ensure that backends
 * catch up before the pages they'll need to read fall out of SLRU cache.
 */
#define QUEUE_CLEANUP_DELAY 4
 
/*
 * Struct describing a listening backend's status
 */
typedef struct QueueBackendStatus
{
    int32       pid;            /* either a PID or InvalidPid */
    Oid         dboid;          /* backend's database OID, or InvalidOid */
    BackendId   nextListener;   /* id of next listener, or InvalidBackendId */
    QueuePosition pos;          /* backend has read queue up to here */
} QueueBackendStatus;
 
/*
 * Shared memory state for LISTEN/NOTIFY (excluding its SLRU stuff)
 *
 * The AsyncQueueControl structure is protected by the NotifyQueueLock and
 * NotifyQueueTailLock.
 *
 * When holding NotifyQueueLock in SHARED mode, backends may only inspect
 * their own entries as well as the head and tail pointers. Consequently we
 * can allow a backend to update its own record while holding only SHARED lock
 * (since no other backend will inspect it).
 *
 * When holding NotifyQueueLock in EXCLUSIVE mode, backends can inspect the
 * entries of other backends and also change the head pointer. When holding
 * both NotifyQueueLock and NotifyQueueTailLock in EXCLUSIVE mode, backends
 * can change the tail pointers.
 *
 * NotifySLRULock is used as the control lock for the pg_notify SLRU buffers.
 * In order to avoid deadlocks, whenever we need multiple locks, we first get
 * NotifyQueueTailLock, then NotifyQueueLock, and lastly NotifySLRULock.
 *
 * Each backend uses the backend[] array entry with index equal to its
 * BackendId (which can range from 1 to MaxBackends).  We rely on this to make
 * SendProcSignal fast.
 *
 * The backend[] array entries for actively-listening backends are threaded
 * together using firstListener and the nextListener links, so that we can
 * scan them without having to iterate over inactive entries.  We keep this
 * list in order by BackendId so that the scan is cache-friendly when there
 * are many active entries.
 */
typedef struct AsyncQueueControl
{
    QueuePosition head;         /* head points to the next free location */
    QueuePosition tail;         /* tail must be <= the queue position of every
                                 * listening backend */
    int         stopPage;       /* oldest unrecycled page; must be <=
                                 * tail.page */
    BackendId   firstListener;  /* id of first listener, or InvalidBackendId */
    TimestampTz lastQueueFillWarn;  /* time of last queue-full msg */
    QueueBackendStatus backend[FLEXIBLE_ARRAY_MEMBER];
    /* backend[0] is not used; used entries are from [1] to [MaxBackends] */
} AsyncQueueControl;
 
static AsyncQueueControl *asyncQueueControl;
 
#define QUEUE_HEAD                  (asyncQueueControl->head)
#define QUEUE_TAIL                  (asyncQueueControl->tail)
#define QUEUE_STOP_PAGE             (asyncQueueControl->stopPage)
#define QUEUE_FIRST_LISTENER        (asyncQueueControl->firstListener)
#define QUEUE_BACKEND_PID(i)        (asyncQueueControl->backend[i].pid)
#define QUEUE_BACKEND_DBOID(i)      (asyncQueueControl->backend[i].dboid)
#define QUEUE_NEXT_LISTENER(i)      (asyncQueueControl->backend[i].nextListener)
#define QUEUE_BACKEND_POS(i)        (asyncQueueControl->backend[i].pos)
 
/*
 * The SLRU buffer area through which we access the notification queue
 */
static SlruCtlData NotifyCtlData;
 
#define NotifyCtl                   (&NotifyCtlData)
#define QUEUE_PAGESIZE              BLCKSZ
#define QUEUE_FULL_WARN_INTERVAL    5000    /* warn at most once every 5s */
 
/*
 * Use segments 0000 through FFFF.  Each contains SLRU_PAGES_PER_SEGMENT pages
 * which gives us the pages from 0 to SLRU_PAGES_PER_SEGMENT * 0x10000 - 1.
 * We could use as many segments as SlruScanDirectory() allows, but this gives
 * us so much space already that it doesn't seem worth the trouble.
 *
 * The most data we can have in the queue at a time is QUEUE_MAX_PAGE/2
 * pages, because more than that would confuse slru.c into thinking there
 * was a wraparound condition.  With the default BLCKSZ this means there
 * can be up to 8GB of queued-and-not-read data.
 *
 * Note: it's possible to redefine QUEUE_MAX_PAGE with a smaller multiple of
 * SLRU_PAGES_PER_SEGMENT, for easier testing of queue-full behaviour.
 */
#define QUEUE_MAX_PAGE          (SLRU_PAGES_PER_SEGMENT * 0x10000 - 1)
 
/*
 * listenChannels identifies the channels we are actually listening to
 * (ie, have committed a LISTEN on).  It is a simple list of channel names,
 * allocated in TopMemoryContext.
 */
static List *listenChannels = NIL;  /* list of C strings */
 
/*
 * State for pending LISTEN/UNLISTEN actions consists of an ordered list of
 * all actions requested in the current transaction.  As explained above,
 * we don't actually change listenChannels until we reach transaction commit.
 *
 * The list is kept in CurTransactionContext.  In subtransactions, each
 * subtransaction has its own list in its own CurTransactionContext, but
 * successful subtransactions attach their lists to their parent's list.
 * Failed subtransactions simply discard their lists.
 */
typedef enum
{
    LISTEN_LISTEN,
    LISTEN_UNLISTEN,
    LISTEN_UNLISTEN_ALL
} ListenActionKind;
 
typedef struct
{
    ListenActionKind action;
    char        channel[FLEXIBLE_ARRAY_MEMBER]; /* nul-terminated string */
} ListenAction;
 
typedef struct ActionList
{
    int         nestingLevel;   /* current transaction nesting depth */
    List       *actions;        /* list of ListenAction structs */
    struct ActionList *upper;   /* details for upper transaction levels */
} ActionList;
 
static ActionList *pendingActions = NULL;
 
/*
 * State for outbound notifies consists of a list of all channels+payloads
 * NOTIFYed in the current transaction.  We do not actually perform a NOTIFY
 * until and unless the transaction commits.  pendingNotifies is NULL if no
 * NOTIFYs have been done in the current (sub) transaction.
 *
 * We discard duplicate notify events issued in the same transaction.
 * Hence, in addition to the list proper (which we need to track the order
 * of the events, since we guarantee to deliver them in order), we build a
 * hash table which we can probe to detect duplicates.  Since building the
 * hash table is somewhat expensive, we do so only once we have at least
 * MIN_HASHABLE_NOTIFIES events queued in the current (sub) transaction;
 * before that we just scan the events linearly.
 *
 * The list is kept in CurTransactionContext.  In subtransactions, each
 * subtransaction has its own list in its own CurTransactionContext, but
 * successful subtransactions add their entries to their parent's list.
 * Failed subtransactions simply discard their lists.  Since these lists
 * are independent, there may be notify events in a subtransaction's list
 * that duplicate events in some ancestor (sub) transaction; we get rid of
 * the dups when merging the subtransaction's list into its parent's.
 *
 * Note: the action and notify lists do not interact within a transaction.
 * In particular, if a transaction does NOTIFY and then LISTEN on the same
 * condition name, it will get a self-notify at commit.  This is a bit odd
 * but is consistent with our historical behavior.
 */
typedef struct Notification
{
    uint16      channel_len;    /* length of channel-name string */
    uint16      payload_len;    /* length of payload string */
    /* null-terminated channel name, then null-terminated payload follow */
    char        data[FLEXIBLE_ARRAY_MEMBER];
} Notification;
 
typedef struct NotificationList
{
    int         nestingLevel;   /* current transaction nesting depth */
    List       *events;         /* list of Notification structs */
    HTAB       *hashtab;        /* hash of NotificationHash structs, or NULL */
    struct NotificationList *upper; /* details for upper transaction levels */
} NotificationList;
 
#define MIN_HASHABLE_NOTIFIES 16    /* threshold to build hashtab */
 
typedef struct NotificationHash
{
    Notification *event;        /* => the actual Notification struct */
} NotificationHash;
 
static NotificationList *pendingNotifies = NULL;
 
/*
 * Inbound notifications are initially processed by HandleNotifyInterrupt(),
 * called from inside a signal handler. That just sets the
 * notifyInterruptPending flag and sets the process
 * latch. ProcessNotifyInterrupt() will then be called whenever it's safe to
 * actually deal with the interrupt.
 */
volatile sig_atomic_t notifyInterruptPending = false;
 
/* True if we've registered an on_shmem_exit cleanup */
static bool unlistenExitRegistered = false;
 
/* True if we're currently registered as a listener in asyncQueueControl */
static bool amRegisteredListener = false;
 
/* have we advanced to a page that's a multiple of QUEUE_CLEANUP_DELAY? */
static bool tryAdvanceTail = false;
 
/* GUC parameter */
bool        Trace_notify = false;
 
/* local function prototypes */
static int  asyncQueuePageDiff(int p, int q);
static bool asyncQueuePagePrecedes(int p, int q);
static void queue_listen(ListenActionKind action, const char *channel);
static void Async_UnlistenOnExit(int code, Datum arg);
static void Exec_ListenPreCommit(void);
static void Exec_ListenCommit(const char *channel);
static void Exec_UnlistenCommit(const char *channel);
static void Exec_UnlistenAllCommit(void);
static bool IsListeningOn(const char *channel);
static void asyncQueueUnregister(void);
static bool asyncQueueIsFull(void);
static bool asyncQueueAdvance(volatile QueuePosition *position, int entryLength);
static void asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe);
static ListCell *asyncQueueAddEntries(ListCell *nextNotify);
static double asyncQueueUsage(void);
static void asyncQueueFillWarning(void);
static void SignalBackends(void);
static void asyncQueueReadAllNotifications(void);
static bool asyncQueueProcessPageEntries(volatile QueuePosition *current,
                                         QueuePosition stop,
                                         char *page_buffer,
                                         Snapshot snapshot);
static void asyncQueueAdvanceTail(void);
static void ProcessIncomingNotify(bool flush);
static bool AsyncExistsPendingNotify(Notification *n);
static void AddEventToPendingNotifies(Notification *n);
static uint32 notification_hash(const void *key, Size keysize);
static int  notification_match(const void *key1, const void *key2, Size keysize);
static void ClearPendingActionsAndNotifies(void);
 
/*
 * Compute the difference between two queue page numbers (i.e., p - q),
 * accounting for wraparound.
 */
static int
asyncQueuePageDiff(int p, int q)
{
    int         diff;
 
    /*
     * We have to compare modulo (QUEUE_MAX_PAGE+1)/2.  Both inputs should be
     * in the range 0..QUEUE_MAX_PAGE.
     */
    Assert(p >= 0 && p <= QUEUE_MAX_PAGE);
    Assert(q >= 0 && q <= QUEUE_MAX_PAGE);
 
    diff = p - q;
    if (diff >= ((QUEUE_MAX_PAGE + 1) / 2))
        diff -= QUEUE_MAX_PAGE + 1;
    else if (diff < -((QUEUE_MAX_PAGE + 1) / 2))
        diff += QUEUE_MAX_PAGE + 1;
    return diff;
}
 
/*
 * Is p < q, accounting for wraparound?
 *
 * Since asyncQueueIsFull() blocks creation of a page that could precede any
 * extant page, we need not assess entries within a page.
 */
static bool
asyncQueuePagePrecedes(int p, int q)
{
    return asyncQueuePageDiff(p, q) < 0;
}
 
/*
 * Report space needed for our shared memory area
 */
Size
AsyncShmemSize(void)
{
    Size        size;
 
    /* This had better match AsyncShmemInit */
    size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
    size = add_size(size, offsetof(AsyncQueueControl, backend));
 
    size = add_size(size, SimpleLruShmemSize(NUM_NOTIFY_BUFFERS, 0));
 
    return size;
}
 
/*
 * Initialize our shared memory area
 */
void
AsyncShmemInit(void)
{
    bool        found;
    Size        size;
 
    /*
     * Create or attach to the AsyncQueueControl structure.
     *
     * The used entries in the backend[] array run from 1 to MaxBackends; the
     * zero'th entry is unused but must be allocated.
     */
    size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
    size = add_size(size, offsetof(AsyncQueueControl, backend));
 
    asyncQueueControl = (AsyncQueueControl *)
        ShmemInitStruct("Async Queue Control", size, &found);
 
    if (!found)
    {
        /* First time through, so initialize it */
        SET_QUEUE_POS(QUEUE_HEAD, 0, 0);
        SET_QUEUE_POS(QUEUE_TAIL, 0, 0);
        QUEUE_STOP_PAGE = 0;
        QUEUE_FIRST_LISTENER = InvalidBackendId;
        asyncQueueControl->lastQueueFillWarn = 0;
        /* zero'th entry won't be used, but let's initialize it anyway */
        for (int i = 0; i <= MaxBackends; i++)
        {
            QUEUE_BACKEND_PID(i) = InvalidPid;
            QUEUE_BACKEND_DBOID(i) = InvalidOid;
            QUEUE_NEXT_LISTENER(i) = InvalidBackendId;
            SET_QUEUE_POS(QUEUE_BACKEND_POS(i), 0, 0);
        }
    }
 
    /*
     * Set up SLRU management of the pg_notify data.
     */
    NotifyCtl->PagePrecedes = asyncQueuePagePrecedes;
    SimpleLruInit(NotifyCtl, "Notify", NUM_NOTIFY_BUFFERS, 0,
                  NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER,
                  SYNC_HANDLER_NONE);
 
    if (!found)
    {
        /*
         * During start or reboot, clean out the pg_notify directory.
         */
        (void) SlruScanDirectory(NotifyCtl, SlruScanDirCbDeleteAll, NULL);
    }
}
 
 
/*
 * pg_notify -
 *    SQL function to send a notification event
 */
Datum
pg_notify(PG_FUNCTION_ARGS)
{
    const char *channel;
    const char *payload;
 
    if (PG_ARGISNULL(0))
        channel = "";
    else
        channel = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
    if (PG_ARGISNULL(1))
        payload = "";
    else
        payload = text_to_cstring(PG_GETARG_TEXT_PP(1));
 
    /* For NOTIFY as a statement, this is checked in ProcessUtility */
    PreventCommandDuringRecovery("NOTIFY");
 
    Async_Notify(channel, payload);
 
    PG_RETURN_VOID();
}
 
 
/*
 * Async_Notify
 *
 *      This is executed by the SQL notify command.
 *
 *      Adds the message to the list of pending notifies.
 *      Actual notification happens during transaction commit.
 *      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 */
void
Async_Notify(const char *channel, const char *payload)
{
    int         my_level = GetCurrentTransactionNestLevel();
    size_t      channel_len;
    size_t      payload_len;
    Notification *n;
    MemoryContext oldcontext;
 
    if (IsParallelWorker())
        elog(ERROR, "cannot send notifications from a parallel worker");
 
    if (Trace_notify)
        elog(DEBUG1, "Async_Notify(%s)", channel);
 
    channel_len = channel ? strlen(channel) : 0;
    payload_len = payload ? strlen(payload) : 0;
 
    /* a channel name must be specified */
    if (channel_len == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("channel name cannot be empty")));
 
    /* enforce length limits */
    if (channel_len >= NAMEDATALEN)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("channel name too long")));
 
    if (payload_len >= NOTIFY_PAYLOAD_MAX_LENGTH)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("payload string too long")));
 
    /*
     * We must construct the Notification entry, even if we end up not using
     * it, in order to compare it cheaply to existing list entries.
     *
     * The notification list needs to live until end of transaction, so store
     * it in the transaction context.
     */
    oldcontext = MemoryContextSwitchTo(CurTransactionContext);
 
    n = (Notification *) palloc(offsetof(Notification, data) +
                                channel_len + payload_len + 2);
    n->channel_len = channel_len;
    n->payload_len = payload_len;
    strcpy(n->data, channel);
    if (payload)
        strcpy(n->data + channel_len + 1, payload);
    else
        n->data[channel_len + 1] = '\0';
 
    if (pendingNotifies == NULL || my_level > pendingNotifies->nestingLevel)
    {
        NotificationList *notifies;
 
        /*
         * First notify event in current (sub)xact. Note that we allocate the
         * NotificationList in TopTransactionContext; the nestingLevel might
         * get changed later by AtSubCommit_Notify.
         */
        notifies = (NotificationList *)
            MemoryContextAlloc(TopTransactionContext,
                               sizeof(NotificationList));
        notifies->nestingLevel = my_level;
        notifies->events = list_make1(n);
        /* We certainly don't need a hashtable yet */
        notifies->hashtab = NULL;
        notifies->upper = pendingNotifies;
        pendingNotifies = notifies;
    }
    else
    {
        /* Now check for duplicates */
        if (AsyncExistsPendingNotify(n))
        {
            /* It's a dup, so forget it */
            pfree(n);
            MemoryContextSwitchTo(oldcontext);
            return;
        }
 
        /* Append more events to existing list */
        AddEventToPendingNotifies(n);
    }
 
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * queue_listen
 *      Common code for listen, unlisten, unlisten all commands.
 *
 *      Adds the request to the list of pending actions.
 *      Actual update of the listenChannels list happens during transaction
 *      commit.
 */
static void
queue_listen(ListenActionKind action, const char *channel)
{
    MemoryContext oldcontext;
    ListenAction *actrec;
    int         my_level = GetCurrentTransactionNestLevel();
 
    /*
     * Unlike Async_Notify, we don't try to collapse out duplicates. It would
     * be too complicated to ensure we get the right interactions of
     * conflicting LISTEN/UNLISTEN/UNLISTEN_ALL, and it's unlikely that there
     * would be any performance benefit anyway in sane applications.
     */
    oldcontext = MemoryContextSwitchTo(CurTransactionContext);
 
    /* space for terminating null is included in sizeof(ListenAction) */
    actrec = (ListenAction *) palloc(offsetof(ListenAction, channel) +
                                     strlen(channel) + 1);
    actrec->action = action;
    strcpy(actrec->channel, channel);
 
    if (pendingActions == NULL || my_level > pendingActions->nestingLevel)
    {
        ActionList *actions;
 
        /*
         * First action in current sub(xact). Note that we allocate the
         * ActionList in TopTransactionContext; the nestingLevel might get
         * changed later by AtSubCommit_Notify.
         */
        actions = (ActionList *)
            MemoryContextAlloc(TopTransactionContext, sizeof(ActionList));
        actions->nestingLevel = my_level;
        actions->actions = list_make1(actrec);
        actions->upper = pendingActions;
        pendingActions = actions;
    }
    else
        pendingActions->actions = lappend(pendingActions->actions, actrec);
 
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * Async_Listen
 *
 *      This is executed by the SQL listen command.
 */
void
Async_Listen(const char *channel)
{
    if (Trace_notify)
        elog(DEBUG1, "Async_Listen(%s,%d)", channel, MyProcPid);
 
    queue_listen(LISTEN_LISTEN, channel);
}
 
/*
 * Async_Unlisten
 *
 *      This is executed by the SQL unlisten command.
 */
void
Async_Unlisten(const char *channel)
{
    if (Trace_notify)
        elog(DEBUG1, "Async_Unlisten(%s,%d)", channel, MyProcPid);
 
    /* If we couldn't possibly be listening, no need to queue anything */
    if (pendingActions == NULL && !unlistenExitRegistered)
        return;
 
    queue_listen(LISTEN_UNLISTEN, channel);
}
 
/*
 * Async_UnlistenAll
 *
 *      This is invoked by UNLISTEN * command, and also at backend exit.
 */
void
Async_UnlistenAll(void)
{
    if (Trace_notify)
        elog(DEBUG1, "Async_UnlistenAll(%d)", MyProcPid);
 
    /* If we couldn't possibly be listening, no need to queue anything */
    if (pendingActions == NULL && !unlistenExitRegistered)
        return;
 
    queue_listen(LISTEN_UNLISTEN_ALL, "");
}
 
/*
 * SQL function: return a set of the channel names this backend is actively
 * listening to.
 *
 * Note: this coding relies on the fact that the listenChannels list cannot
 * change within a transaction.
 */
Datum
pg_listening_channels(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
 
    /* stuff done only on the first call of the function */
    if (SRF_IS_FIRSTCALL())
    {
        /* create a function context for cross-call persistence */
        funcctx = SRF_FIRSTCALL_INIT();
    }
 
    /* stuff done on every call of the function */
    funcctx = SRF_PERCALL_SETUP();
 
    if (funcctx->call_cntr < list_length(listenChannels))
    {
        char       *channel = (char *) list_nth(listenChannels,
                                                funcctx->call_cntr);
 
        SRF_RETURN_NEXT(funcctx, CStringGetTextDatum(channel));
    }
 
    SRF_RETURN_DONE(funcctx);
}
 
/*
 * Async_UnlistenOnExit
 *
 * This is executed at backend exit if we have done any LISTENs in this
 * backend.  It might not be necessary anymore, if the user UNLISTENed
 * everything, but we don't try to detect that case.
 */
static void
Async_UnlistenOnExit(int code, Datum arg)
{
    Exec_UnlistenAllCommit();
    asyncQueueUnregister();
}
 
/*
 * AtPrepare_Notify
 *
 *      This is called at the prepare phase of a two-phase
 *      transaction.  Save the state for possible commit later.
 */
void
AtPrepare_Notify(void)
{
    /* It's not allowed to have any pending LISTEN/UNLISTEN/NOTIFY actions */
    if (pendingActions || pendingNotifies)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot PREPARE a transaction that has executed LISTEN, UNLISTEN, or NOTIFY")));
}
 
/*
 * PreCommit_Notify
 *
 *      This is called at transaction commit, before actually committing to
 *      clog.
 *
 *      If there are pending LISTEN actions, make sure we are listed in the
 *      shared-memory listener array.  This must happen before commit to
 *      ensure we don't miss any notifies from transactions that commit
 *      just after ours.
 *
 *      If there are outbound notify requests in the pendingNotifies list,
 *      add them to the global queue.  We do that before commit so that
 *      we can still throw error if we run out of queue space.
 */
void
PreCommit_Notify(void)
{
    ListCell   *p;
 
    if (!pendingActions && !pendingNotifies)
        return;                 /* no relevant statements in this xact */
 
    if (Trace_notify)
        elog(DEBUG1, "PreCommit_Notify");
 
    /* Preflight for any pending listen/unlisten actions */
    if (pendingActions != NULL)
    {
        foreach(p, pendingActions->actions)
        {
            ListenAction *actrec = (ListenAction *) lfirst(p);
 
            switch (actrec->action)
            {
                case LISTEN_LISTEN:
                    Exec_ListenPreCommit();
                    break;
                case LISTEN_UNLISTEN:
                    /* there is no Exec_UnlistenPreCommit() */
                    break;
                case LISTEN_UNLISTEN_ALL:
                    /* there is no Exec_UnlistenAllPreCommit() */
                    break;
            }
        }
    }
 
    /* Queue any pending notifies (must happen after the above) */
    if (pendingNotifies)
    {
        ListCell   *nextNotify;
 
        /*
         * Make sure that we have an XID assigned to the current transaction.
         * GetCurrentTransactionId is cheap if we already have an XID, but not
         * so cheap if we don't, and we'd prefer not to do that work while
         * holding NotifyQueueLock.
         */
        (void) GetCurrentTransactionId();
 
        /*
         * Serialize writers by acquiring a special lock that we hold till
         * after commit.  This ensures that queue entries appear in commit
         * order, and in particular that there are never uncommitted queue
         * entries ahead of committed ones, so an uncommitted transaction
         * can't block delivery of deliverable notifications.
         *
         * We use a heavyweight lock so that it'll automatically be released
         * after either commit or abort.  This also allows deadlocks to be
         * detected, though really a deadlock shouldn't be possible here.
         *
         * The lock is on "database 0", which is pretty ugly but it doesn't
         * seem worth inventing a special locktag category just for this.
         * (Historical note: before PG 9.0, a similar lock on "database 0" was
         * used by the flatfiles mechanism.)
         */
        LockSharedObject(DatabaseRelationId, InvalidOid, 0,
                         AccessExclusiveLock);
 
        /* Now push the notifications into the queue */
        nextNotify = list_head(pendingNotifies->events);
        while (nextNotify != NULL)
        {
            /*
             * Add the pending notifications to the queue.  We acquire and
             * release NotifyQueueLock once per page, which might be overkill
             * but it does allow readers to get in while we're doing this.
             *
             * A full queue is very uncommon and should really not happen,
             * given that we have so much space available in the SLRU pages.
             * Nevertheless we need to deal with this possibility. Note that
             * when we get here we are in the process of committing our
             * transaction, but we have not yet committed to clog, so at this
             * point in time we can still roll the transaction back.
             */
            LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
            asyncQueueFillWarning();
            if (asyncQueueIsFull())
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("too many notifications in the NOTIFY queue")));
            nextNotify = asyncQueueAddEntries(nextNotify);
            LWLockRelease(NotifyQueueLock);
        }
 
        /* Note that we don't clear pendingNotifies; AtCommit_Notify will. */
    }
}
 
/*
 * AtCommit_Notify
 *
 *      This is called at transaction commit, after committing to clog.
 *
 *      Update listenChannels and clear transaction-local state.
 *
 *      If we issued any notifications in the transaction, send signals to
 *      listening backends (possibly including ourselves) to process them.
 *      Also, if we filled enough queue pages with new notifies, try to
 *      advance the queue tail pointer.
 */
void
AtCommit_Notify(void)
{
    ListCell   *p;
 
    /*
     * Allow transactions that have not executed LISTEN/UNLISTEN/NOTIFY to
     * return as soon as possible
     */
    if (!pendingActions && !pendingNotifies)
        return;
 
    if (Trace_notify)
        elog(DEBUG1, "AtCommit_Notify");
 
    /* Perform any pending listen/unlisten actions */
    if (pendingActions != NULL)
    {
        foreach(p, pendingActions->actions)
        {
            ListenAction *actrec = (ListenAction *) lfirst(p);
 
            switch (actrec->action)
            {
                case LISTEN_LISTEN:
                    Exec_ListenCommit(actrec->channel);
                    break;
                case LISTEN_UNLISTEN:
                    Exec_UnlistenCommit(actrec->channel);
                    break;
                case LISTEN_UNLISTEN_ALL:
                    Exec_UnlistenAllCommit();
                    break;
            }
        }
    }
 
    /* If no longer listening to anything, get out of listener array */
    if (amRegisteredListener && listenChannels == NIL)
        asyncQueueUnregister();
 
    /*
     * Send signals to listening backends.  We need do this only if there are
     * pending notifies, which were previously added to the shared queue by
     * PreCommit_Notify().
     */
    if (pendingNotifies != NULL)
        SignalBackends();
 
    /*
     * If it's time to try to advance the global tail pointer, do that.
     *
     * (It might seem odd to do this in the sender, when more than likely the
     * listeners won't yet have read the messages we just sent.  However,
     * there's less contention if only the sender does it, and there is little
     * need for urgency in advancing the global tail.  So this typically will
     * be clearing out messages that were sent some time ago.)
     */
    if (tryAdvanceTail)
    {
        tryAdvanceTail = false;
        asyncQueueAdvanceTail();
    }
 
    /* And clean up */
    ClearPendingActionsAndNotifies();
}
 
/*
 * Exec_ListenPreCommit --- subroutine for PreCommit_Notify
 *
 * This function must make sure we are ready to catch any incoming messages.
 */
static void
Exec_ListenPreCommit(void)
{
    QueuePosition head;
    QueuePosition max;
    BackendId   prevListener;
 
    /*
     * Nothing to do if we are already listening to something, nor if we
     * already ran this routine in this transaction.
     */
    if (amRegisteredListener)
        return;
 
    if (Trace_notify)
        elog(DEBUG1, "Exec_ListenPreCommit(%d)", MyProcPid);
 
    /*
     * Before registering, make sure we will unlisten before dying. (Note:
     * this action does not get undone if we abort later.)
     */
    if (!unlistenExitRegistered)
    {
        before_shmem_exit(Async_UnlistenOnExit, 0);
        unlistenExitRegistered = true;
    }
 
    /*
     * This is our first LISTEN, so establish our pointer.
     *
     * We set our pointer to the global tail pointer and then move it forward
     * over already-committed notifications.  This ensures we cannot miss any
     * not-yet-committed notifications.  We might get a few more but that
     * doesn't hurt.
     *
     * In some scenarios there might be a lot of committed notifications that
     * have not yet been pruned away (because some backend is being lazy about
     * reading them).  To reduce our startup time, we can look at other
     * backends and adopt the maximum "pos" pointer of any backend that's in
     * our database; any notifications it's already advanced over are surely
     * committed and need not be re-examined by us.  (We must consider only
     * backends connected to our DB, because others will not have bothered to
     * check committed-ness of notifications in our DB.)
     *
     * We need exclusive lock here so we can look at other backends' entries
     * and manipulate the list links.
     */
    LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
    head = QUEUE_HEAD;
    max = QUEUE_TAIL;
    prevListener = InvalidBackendId;
    for (BackendId i = QUEUE_FIRST_LISTENER; i > 0; i = QUEUE_NEXT_LISTENER(i))
    {
        if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
            max = QUEUE_POS_MAX(max, QUEUE_BACKEND_POS(i));
        /* Also find last listening backend before this one */
        if (i < MyBackendId)
            prevListener = i;
    }
    QUEUE_BACKEND_POS(MyBackendId) = max;
    QUEUE_BACKEND_PID(MyBackendId) = MyProcPid;
    QUEUE_BACKEND_DBOID(MyBackendId) = MyDatabaseId;
    /* Insert backend into list of listeners at correct position */
    if (prevListener > 0)
    {
        QUEUE_NEXT_LISTENER(MyBackendId) = QUEUE_NEXT_LISTENER(prevListener);
        QUEUE_NEXT_LISTENER(prevListener) = MyBackendId;
    }
    else
    {
        QUEUE_NEXT_LISTENER(MyBackendId) = QUEUE_FIRST_LISTENER;
        QUEUE_FIRST_LISTENER = MyBackendId;
    }
    LWLockRelease(NotifyQueueLock);
 
    /* Now we are listed in the global array, so remember we're listening */
    amRegisteredListener = true;
 
    /*
     * Try to move our pointer forward as far as possible.  This will skip
     * over already-committed notifications, which we want to do because they
     * might be quite stale.  Note that we are not yet listening on anything,
     * so we won't deliver such notifications to our frontend.  Also, although
     * our transaction might have executed NOTIFY, those message(s) aren't
     * queued yet so we won't skip them here.
     */
    if (!QUEUE_POS_EQUAL(max, head))
        asyncQueueReadAllNotifications();
}
 
/*
 * Exec_ListenCommit --- subroutine for AtCommit_Notify
 *
 * Add the channel to the list of channels we are listening on.
 */
static void
Exec_ListenCommit(const char *channel)
{
    MemoryContext oldcontext;
 
    /* Do nothing if we are already listening on this channel */
    if (IsListeningOn(channel))
        return;
 
    /*
     * Add the new channel name to listenChannels.
     *
     * XXX It is theoretically possible to get an out-of-memory failure here,
     * which would be bad because we already committed.  For the moment it
     * doesn't seem worth trying to guard against that, but maybe improve this
     * later.
     */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
    listenChannels = lappend(listenChannels, pstrdup(channel));
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * Exec_UnlistenCommit --- subroutine for AtCommit_Notify
 *
 * Remove the specified channel name from listenChannels.
 */
static void
Exec_UnlistenCommit(const char *channel)
{
    ListCell   *q;
 
    if (Trace_notify)
        elog(DEBUG1, "Exec_UnlistenCommit(%s,%d)", channel, MyProcPid);
 
    foreach(q, listenChannels)
    {
        char       *lchan = (char *) lfirst(q);
 
        if (strcmp(lchan, channel) == 0)
        {
            listenChannels = foreach_delete_current(listenChannels, q);
            pfree(lchan);
            break;
        }
    }
 
    /*
     * We do not complain about unlistening something not being listened;
     * should we?
     */
}
 
/*
 * Exec_UnlistenAllCommit --- subroutine for AtCommit_Notify
 *
 *      Unlisten on all channels for this backend.
 */
static void
Exec_UnlistenAllCommit(void)
{
    if (Trace_notify)
        elog(DEBUG1, "Exec_UnlistenAllCommit(%d)", MyProcPid);
 
    list_free_deep(listenChannels);
    listenChannels = NIL;
}
 
/*
 * Test whether we are actively listening on the given channel name.
 *
 * Note: this function is executed for every notification found in the queue.
 * Perhaps it is worth further optimization, eg convert the list to a sorted
 * array so we can binary-search it.  In practice the list is likely to be
 * fairly short, though.
 */
static bool
IsListeningOn(const char *channel)
{
    ListCell   *p;
 
    foreach(p, listenChannels)
    {
        char       *lchan = (char *) lfirst(p);
 
        if (strcmp(lchan, channel) == 0)
            return true;
    }
    return false;
}
 
/*
 * Remove our entry from the listeners array when we are no longer listening
 * on any channel.  NB: must not fail if we're already not listening.
 */
static void
asyncQueueUnregister(void)
{
    Assert(listenChannels == NIL);  /* else caller error */
 
    if (!amRegisteredListener)  /* nothing to do */
        return;
 
    /*
     * Need exclusive lock here to manipulate list links.
     */
    LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
    /* Mark our entry as invalid */
    QUEUE_BACKEND_PID(MyBackendId) = InvalidPid;
    QUEUE_BACKEND_DBOID(MyBackendId) = InvalidOid;
    /* and remove it from the list */
    if (QUEUE_FIRST_LISTENER == MyBackendId)
        QUEUE_FIRST_LISTENER = QUEUE_NEXT_LISTENER(MyBackendId);
    else
    {
        for (BackendId i = QUEUE_FIRST_LISTENER; i > 0; i = QUEUE_NEXT_LISTENER(i))
        {
            if (QUEUE_NEXT_LISTENER(i) == MyBackendId)
            {
                QUEUE_NEXT_LISTENER(i) = QUEUE_NEXT_LISTENER(MyBackendId);
                break;
            }
        }
    }
    QUEUE_NEXT_LISTENER(MyBackendId) = InvalidBackendId;
    LWLockRelease(NotifyQueueLock);
 
    /* mark ourselves as no longer listed in the global array */
    amRegisteredListener = false;
}
 
/*
 * Test whether there is room to insert more notification messages.
 *
 * Caller must hold at least shared NotifyQueueLock.
 */
static bool
asyncQueueIsFull(void)
{
    int         nexthead;
    int         boundary;
 
    /*
     * The queue is full if creating a new head page would create a page that
     * logically precedes the current global tail pointer, ie, the head
     * pointer would wrap around compared to the tail.  We cannot create such
     * a head page for fear of confusing slru.c.  For safety we round the tail
     * pointer back to a segment boundary (truncation logic in
     * asyncQueueAdvanceTail does not do this, so doing it here is optional).
     *
     * Note that this test is *not* dependent on how much space there is on
     * the current head page.  This is necessary because asyncQueueAddEntries
     * might try to create the next head page in any case.
     */
    nexthead = QUEUE_POS_PAGE(QUEUE_HEAD) + 1;
    if (nexthead > QUEUE_MAX_PAGE)
        nexthead = 0;           /* wrap around */
    boundary = QUEUE_STOP_PAGE;
    boundary -= boundary % SLRU_PAGES_PER_SEGMENT;
    return asyncQueuePagePrecedes(nexthead, boundary);
}
 
/*
 * Advance the QueuePosition to the next entry, assuming that the current
 * entry is of length entryLength.  If we jump to a new page the function
 * returns true, else false.
 */
static bool
asyncQueueAdvance(volatile QueuePosition *position, int entryLength)
{
    int         pageno = QUEUE_POS_PAGE(*position);
    int         offset = QUEUE_POS_OFFSET(*position);
    bool        pageJump = false;
 
    /*
     * Move to the next writing position: First jump over what we have just
     * written or read.
     */
    offset += entryLength;
    Assert(offset <= QUEUE_PAGESIZE);
 
    /*
     * In a second step check if another entry can possibly be written to the
     * page. If so, stay here, we have reached the next position. If not, then
     * we need to move on to the next page.
     */
    if (offset + QUEUEALIGN(AsyncQueueEntryEmptySize) > QUEUE_PAGESIZE)
    {
        pageno++;
        if (pageno > QUEUE_MAX_PAGE)
            pageno = 0;         /* wrap around */
        offset = 0;
        pageJump = true;
    }
 
    SET_QUEUE_POS(*position, pageno, offset);
    return pageJump;
}
 
/*
 * Fill the AsyncQueueEntry at *qe with an outbound notification message.
 */
static void
asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe)
{
    size_t      channellen = n->channel_len;
    size_t      payloadlen = n->payload_len;
    int         entryLength;
 
    Assert(channellen < NAMEDATALEN);
    Assert(payloadlen < NOTIFY_PAYLOAD_MAX_LENGTH);
 
    /* The terminators are already included in AsyncQueueEntryEmptySize */
    entryLength = AsyncQueueEntryEmptySize + payloadlen + channellen;
    entryLength = QUEUEALIGN(entryLength);
    qe->length = entryLength;
    qe->dboid = MyDatabaseId;
    qe->xid = GetCurrentTransactionId();
    qe->srcPid = MyProcPid;
    memcpy(qe->data, n->data, channellen + payloadlen + 2);
}
 
/*
 * Add pending notifications to the queue.
 *
 * We go page by page here, i.e. we stop once we have to go to a new page but
 * we will be called again and then fill that next page. If an entry does not
 * fit into the current page, we write a dummy entry with an InvalidOid as the
 * database OID in order to fill the page. So every page is always used up to
 * the last byte which simplifies reading the page later.
 *
 * We are passed the list cell (in pendingNotifies->events) containing the next
 * notification to write and return the first still-unwritten cell back.
 * Eventually we will return NULL indicating all is done.
 *
 * We are holding NotifyQueueLock already from the caller and grab
 * NotifySLRULock locally in this function.
 */
static ListCell *
asyncQueueAddEntries(ListCell *nextNotify)
{
    AsyncQueueEntry qe;
    QueuePosition queue_head;
    int         pageno;
    int         offset;
    int         slotno;
 
    /* We hold both NotifyQueueLock and NotifySLRULock during this operation */
    LWLockAcquire(NotifySLRULock, LW_EXCLUSIVE);
 
    /*
     * We work with a local copy of QUEUE_HEAD, which we write back to shared
     * memory upon exiting.  The reason for this is that if we have to advance
     * to a new page, SimpleLruZeroPage might fail (out of disk space, for
     * instance), and we must not advance QUEUE_HEAD if it does.  (Otherwise,
     * subsequent insertions would try to put entries into a page that slru.c
     * thinks doesn't exist yet.)  So, use a local position variable.  Note
     * that if we do fail, any already-inserted queue entries are forgotten;
     * this is okay, since they'd be useless anyway after our transaction
     * rolls back.
     */
    queue_head = QUEUE_HEAD;
 
    /*
     * If this is the first write since the postmaster started, we need to
     * initialize the first page of the async SLRU.  Otherwise, the current
     * page should be initialized already, so just fetch it.
     *
     * (We could also take the first path when the SLRU position has just
     * wrapped around, but re-zeroing the page is harmless in that case.)
     */
    pageno = QUEUE_POS_PAGE(queue_head);
    if (QUEUE_POS_IS_ZERO(queue_head))
        slotno = SimpleLruZeroPage(NotifyCtl, pageno);
    else
        slotno = SimpleLruReadPage(NotifyCtl, pageno, true,
                                   InvalidTransactionId);
 
    /* Note we mark the page dirty before writing in it */
    NotifyCtl->shared->page_dirty[slotno] = true;
 
    while (nextNotify != NULL)
    {
        Notification *n = (Notification *) lfirst(nextNotify);
 
        /* Construct a valid queue entry in local variable qe */
        asyncQueueNotificationToEntry(n, &qe);
 
        offset = QUEUE_POS_OFFSET(queue_head);
 
        /* Check whether the entry really fits on the current page */
        if (offset + qe.length <= QUEUE_PAGESIZE)
        {
            /* OK, so advance nextNotify past this item */
            nextNotify = lnext(pendingNotifies->events, nextNotify);
        }
        else
        {
            /*
             * Write a dummy entry to fill up the page. Actually readers will
             * only check dboid and since it won't match any reader's database
             * OID, they will ignore this entry and move on.
             */
            qe.length = QUEUE_PAGESIZE - offset;
            qe.dboid = InvalidOid;
            qe.data[0] = '\0';  /* empty channel */
            qe.data[1] = '\0';  /* empty payload */
        }
 
        /* Now copy qe into the shared buffer page */
        memcpy(NotifyCtl->shared->page_buffer[slotno] + offset,
               &qe,
               qe.length);
 
        /* Advance queue_head appropriately, and detect if page is full */
        if (asyncQueueAdvance(&(queue_head), qe.length))
        {
            /*
             * Page is full, so we're done here, but first fill the next page
             * with zeroes.  The reason to do this is to ensure that slru.c's
             * idea of the head page is always the same as ours, which avoids
             * boundary problems in SimpleLruTruncate.  The test in
             * asyncQueueIsFull() ensured that there is room to create this
             * page without overrunning the queue.
             */
            slotno = SimpleLruZeroPage(NotifyCtl, QUEUE_POS_PAGE(queue_head));
 
            /*
             * If the new page address is a multiple of QUEUE_CLEANUP_DELAY,
             * set flag to remember that we should try to advance the tail
             * pointer (we don't want to actually do that right here).
             */
            if (QUEUE_POS_PAGE(queue_head) % QUEUE_CLEANUP_DELAY == 0)
                tryAdvanceTail = true;
 
            /* And exit the loop */
            break;
        }
    }
 
    /* Success, so update the global QUEUE_HEAD */
    QUEUE_HEAD = queue_head;
 
    LWLockRelease(NotifySLRULock);
 
    return nextNotify;
}
 
/*
 * SQL function to return the fraction of the notification queue currently
 * occupied.
 */
Datum
pg_notification_queue_usage(PG_FUNCTION_ARGS)
{
    double      usage;
 
    /* Advance the queue tail so we don't report a too-large result */
    asyncQueueAdvanceTail();
 
    LWLockAcquire(NotifyQueueLock, LW_SHARED);
    usage = asyncQueueUsage();
    LWLockRelease(NotifyQueueLock);
 
    PG_RETURN_FLOAT8(usage);
}
 
/*
 * Return the fraction of the queue that is currently occupied.
 *
 * The caller must hold NotifyQueueLock in (at least) shared mode.
 *
 * Note: we measure the distance to the logical tail page, not the physical
 * tail page.  In some sense that's wrong, but the relative position of the
 * physical tail is affected by details such as SLRU segment boundaries,
 * so that a result based on that is unpleasantly unstable.
 */
static double
asyncQueueUsage(void)
{
    int         headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
    int         tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
    int         occupied;
 
    occupied = headPage - tailPage;
 
    if (occupied == 0)
        return (double) 0;      /* fast exit for common case */
 
    if (occupied < 0)
    {
        /* head has wrapped around, tail not yet */
        occupied += QUEUE_MAX_PAGE + 1;
    }
 
    return (double) occupied / (double) ((QUEUE_MAX_PAGE + 1) / 2);
}
 
/*
 * Check whether the queue is at least half full, and emit a warning if so.
 *
 * This is unlikely given the size of the queue, but possible.
 * The warnings show up at most once every QUEUE_FULL_WARN_INTERVAL.
 *
 * Caller must hold exclusive NotifyQueueLock.
 */
static void
asyncQueueFillWarning(void)
{
    double      fillDegree;
    TimestampTz t;
 
    fillDegree = asyncQueueUsage();
    if (fillDegree < 0.5)
        return;
 
    t = GetCurrentTimestamp();
 
    if (TimestampDifferenceExceeds(asyncQueueControl->lastQueueFillWarn,
                                   t, QUEUE_FULL_WARN_INTERVAL))
    {
        QueuePosition min = QUEUE_HEAD;
        int32       minPid = InvalidPid;
 
        for (BackendId i = QUEUE_FIRST_LISTENER; i > 0; i = QUEUE_NEXT_LISTENER(i))
        {
            Assert(QUEUE_BACKEND_PID(i) != InvalidPid);
            min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
            if (QUEUE_POS_EQUAL(min, QUEUE_BACKEND_POS(i)))
                minPid = QUEUE_BACKEND_PID(i);
        }
 
        ereport(WARNING,
                (errmsg("NOTIFY queue is %.0f%% full", fillDegree * 100),
                 (minPid != InvalidPid ?
                  errdetail("The server process with PID %d is among those with the oldest transactions.", minPid)
                  : 0),
                 (minPid != InvalidPid ?
                  errhint("The NOTIFY queue cannot be emptied until that process ends its current transaction.")
                  : 0)));
 
        asyncQueueControl->lastQueueFillWarn = t;
    }
}
 
/*
 * Send signals to listening backends.
 *
 * Normally we signal only backends in our own database, since only those
 * backends could be interested in notifies we send.  However, if there's
 * notify traffic in our database but no traffic in another database that
 * does have listener(s), those listeners will fall further and further
 * behind.  Waken them anyway if they're far enough behind, so that they'll
 * advance their queue position pointers, allowing the global tail to advance.
 *
 * Since we know the BackendId and the Pid the signaling is quite cheap.
 *
 * This is called during CommitTransaction(), so it's important for it
 * to have very low probability of failure.
 */
static void
SignalBackends(void)
{
    int32      *pids;
    BackendId  *ids;
    int         count;
 
    /*
     * Identify backends that we need to signal.  We don't want to send
     * signals while holding the NotifyQueueLock, so this loop just builds a
     * list of target PIDs.
     *
     * XXX in principle these pallocs could fail, which would be bad. Maybe
     * preallocate the arrays?  They're not that large, though.
     */
    pids = (int32 *) palloc(MaxBackends * sizeof(int32));
    ids = (BackendId *) palloc(MaxBackends * sizeof(BackendId));
    count = 0;
 
    LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
    for (BackendId i = QUEUE_FIRST_LISTENER; i > 0; i = QUEUE_NEXT_LISTENER(i))
    {
        int32       pid = QUEUE_BACKEND_PID(i);
        QueuePosition pos;
 
        Assert(pid != InvalidPid);
        pos = QUEUE_BACKEND_POS(i);
        if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
        {
            /*
             * Always signal listeners in our own database, unless they're
             * already caught up (unlikely, but possible).
             */
            if (QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
                continue;
        }
        else
        {
            /*
             * Listeners in other databases should be signaled only if they
             * are far behind.
             */
            if (asyncQueuePageDiff(QUEUE_POS_PAGE(QUEUE_HEAD),
                                   QUEUE_POS_PAGE(pos)) < QUEUE_CLEANUP_DELAY)
                continue;
        }
        /* OK, need to signal this one */
        pids[count] = pid;
        ids[count] = i;
        count++;
    }
    LWLockRelease(NotifyQueueLock);
 
    /* Now send signals */
    for (int i = 0; i < count; i++)
    {
        int32       pid = pids[i];
 
        /*
         * If we are signaling our own process, no need to involve the kernel;
         * just set the flag directly.
         */
        if (pid == MyProcPid)
        {
            notifyInterruptPending = true;
            continue;
        }
 
        /*
         * Note: assuming things aren't broken, a signal failure here could
         * only occur if the target backend exited since we released
         * NotifyQueueLock; which is unlikely but certainly possible. So we
         * just log a low-level debug message if it happens.
         */
        if (SendProcSignal(pid, PROCSIG_NOTIFY_INTERRUPT, ids[i]) < 0)
            elog(DEBUG3, "could not signal backend with PID %d: %m", pid);
    }
 
    pfree(pids);
    pfree(ids);
}
 
/*
 * AtAbort_Notify
 *
 *  This is called at transaction abort.
 *
 *  Gets rid of pending actions and outbound notifies that we would have
 *  executed if the transaction got committed.
 */
void
AtAbort_Notify(void)
{
    /*
     * If we LISTEN but then roll back the transaction after PreCommit_Notify,
     * we have registered as a listener but have not made any entry in
     * listenChannels.  In that case, deregister again.
     */
    if (amRegisteredListener && listenChannels == NIL)
        asyncQueueUnregister();
 
    /* And clean up */
    ClearPendingActionsAndNotifies();
}
 
/*
 * AtSubCommit_Notify() --- Take care of subtransaction commit.
 *
 * Reassign all items in the pending lists to the parent transaction.
 */
void
AtSubCommit_Notify(void)
{
    int         my_level = GetCurrentTransactionNestLevel();
 
    /* If there are actions at our nesting level, we must reparent them. */
    if (pendingActions != NULL &&
        pendingActions->nestingLevel >= my_level)
    {
        if (pendingActions->upper == NULL ||
            pendingActions->upper->nestingLevel < my_level - 1)
        {
            /* nothing to merge; give the whole thing to the parent */
            --pendingActions->nestingLevel;
        }
        else
        {
            ActionList *childPendingActions = pendingActions;
 
            pendingActions = pendingActions->upper;
 
            /*
             * Mustn't try to eliminate duplicates here --- see queue_listen()
             */
            pendingActions->actions =
                list_concat(pendingActions->actions,
                            childPendingActions->actions);
            pfree(childPendingActions);
        }
    }
 
    /* If there are notifies at our nesting level, we must reparent them. */
    if (pendingNotifies != NULL &&
        pendingNotifies->nestingLevel >= my_level)
    {
        Assert(pendingNotifies->nestingLevel == my_level);
 
        if (pendingNotifies->upper == NULL ||
            pendingNotifies->upper->nestingLevel < my_level - 1)
        {
            /* nothing to merge; give the whole thing to the parent */
            --pendingNotifies->nestingLevel;
        }
        else
        {
            /*
             * Formerly, we didn't bother to eliminate duplicates here, but
             * now we must, else we fall foul of "Assert(!found)", either here
             * or during a later attempt to build the parent-level hashtable.
             */
            NotificationList *childPendingNotifies = pendingNotifies;
            ListCell   *l;
 
            pendingNotifies = pendingNotifies->upper;
            /* Insert all the subxact's events into parent, except for dups */
            foreach(l, childPendingNotifies->events)
            {
                Notification *childn = (Notification *) lfirst(l);
 
                if (!AsyncExistsPendingNotify(childn))
                    AddEventToPendingNotifies(childn);
            }
            pfree(childPendingNotifies);
        }
    }
}
 
/*
 * AtSubAbort_Notify() --- Take care of subtransaction abort.
 */
void
AtSubAbort_Notify(void)
{
    int         my_level = GetCurrentTransactionNestLevel();
 
    /*
     * All we have to do is pop the stack --- the actions/notifies made in
     * this subxact are no longer interesting, and the space will be freed
     * when CurTransactionContext is recycled. We still have to free the
     * ActionList and NotificationList objects themselves, though, because
     * those are allocated in TopTransactionContext.
     *
     * Note that there might be no entries at all, or no entries for the
     * current subtransaction level, either because none were ever created, or
     * because we reentered this routine due to trouble during subxact abort.
     */
    while (pendingActions != NULL &&
           pendingActions->nestingLevel >= my_level)
    {
        ActionList *childPendingActions = pendingActions;
 
        pendingActions = pendingActions->upper;
        pfree(childPendingActions);
    }
 
    while (pendingNotifies != NULL &&
           pendingNotifies->nestingLevel >= my_level)
    {
        NotificationList *childPendingNotifies = pendingNotifies;
 
        pendingNotifies = pendingNotifies->upper;
        pfree(childPendingNotifies);
    }
}
 
/*
 * HandleNotifyInterrupt
 *
 *      Signal handler portion of interrupt handling. Let the backend know
 *      that there's a pending notify interrupt. If we're currently reading
 *      from the client, this will interrupt the read and
 *      ProcessClientReadInterrupt() will call ProcessNotifyInterrupt().
 */
void
HandleNotifyInterrupt(void)
{
    /*
     * Note: this is called by a SIGNAL HANDLER. You must be very wary what
     * you do here.
     */
 
    /* signal that work needs to be done */
    notifyInterruptPending = true;
 
    /* make sure the event is processed in due course */
    SetLatch(MyLatch);
}
 
/*
 * ProcessNotifyInterrupt
 *
 *      This is called if we see notifyInterruptPending set, just before
 *      transmitting ReadyForQuery at the end of a frontend command, and
 *      also if a notify signal occurs while reading from the frontend.
 *      HandleNotifyInterrupt() will cause the read to be interrupted
 *      via the process's latch, and this routine will get called.
 *      If we are truly idle (ie, *not* inside a transaction block),
 *      process the incoming notifies.
 *
 *      If "flush" is true, force any frontend messages out immediately.
 *      This can be false when being called at the end of a frontend command,
 *      since we'll flush after sending ReadyForQuery.
 */
void
ProcessNotifyInterrupt(bool flush)
{
    if (IsTransactionOrTransactionBlock())
        return;                 /* not really idle */
 
    /* Loop in case another signal arrives while sending messages */
    while (notifyInterruptPending)
        ProcessIncomingNotify(flush);
}
 
 
/*
 * Read all pending notifications from the queue, and deliver appropriate
 * ones to my frontend.  Stop when we reach queue head or an uncommitted
 * notification.
 */
static void
asyncQueueReadAllNotifications(void)
{
    volatile QueuePosition pos;
    QueuePosition head;
    Snapshot    snapshot;
 
    /* page_buffer must be adequately aligned, so use a union */
    union
    {
        char        buf[QUEUE_PAGESIZE];
        AsyncQueueEntry align;
    }           page_buffer;
 
    /* Fetch current state */
    LWLockAcquire(NotifyQueueLock, LW_SHARED);
    /* Assert checks that we have a valid state entry */
    Assert(MyProcPid == QUEUE_BACKEND_PID(MyBackendId));
    pos = QUEUE_BACKEND_POS(MyBackendId);
    head = QUEUE_HEAD;
    LWLockRelease(NotifyQueueLock);
 
    if (QUEUE_POS_EQUAL(pos, head))
    {
        /* Nothing to do, we have read all notifications already. */
        return;
    }
 
    /*----------
     * Get snapshot we'll use to decide which xacts are still in progress.
     * This is trickier than it might seem, because of race conditions.
     * Consider the following example:
     *
     * Backend 1:                    Backend 2:
     *
     * transaction starts
     * UPDATE foo SET ...;
     * NOTIFY foo;
     * commit starts
     * queue the notify message
     *                               transaction starts
     *                               LISTEN foo;  -- first LISTEN in session
     *                               SELECT * FROM foo WHERE ...;
     * commit to clog
     *                               commit starts
     *                               add backend 2 to array of listeners
     *                               advance to queue head (this code)
     *                               commit to clog
     *
     * Transaction 2's SELECT has not seen the UPDATE's effects, since that
     * wasn't committed yet.  Ideally we'd ensure that client 2 would
     * eventually get transaction 1's notify message, but there's no way
     * to do that; until we're in the listener array, there's no guarantee
     * that the notify message doesn't get removed from the queue.
     *
     * Therefore the coding technique transaction 2 is using is unsafe:
     * applications must commit a LISTEN before inspecting database state,
     * if they want to ensure they will see notifications about subsequent
     * changes to that state.
     *
     * What we do guarantee is that we'll see all notifications from
     * transactions committing after the snapshot we take here.
     * Exec_ListenPreCommit has already added us to the listener array,
     * so no not-yet-committed messages can be removed from the queue
     * before we see them.
     *----------
     */
    snapshot = RegisterSnapshot(GetLatestSnapshot());
 
    /*
     * It is possible that we fail while trying to send a message to our
     * frontend (for example, because of encoding conversion failure).  If
     * that happens it is critical that we not try to send the same message
     * over and over again.  Therefore, we place a PG_TRY block here that will
     * forcibly advance our queue position before we lose control to an error.
     * (We could alternatively retake NotifyQueueLock and move the position
     * before handling each individual message, but that seems like too much
     * lock traffic.)
     */
    PG_TRY();
    {
        bool        reachedStop;
 
        do
        {
            int         curpage = QUEUE_POS_PAGE(pos);
            int         curoffset = QUEUE_POS_OFFSET(pos);
            int         slotno;
            int         copysize;
 
            /*
             * We copy the data from SLRU into a local buffer, so as to avoid
             * holding the NotifySLRULock while we are examining the entries
             * and possibly transmitting them to our frontend.  Copy only the
             * part of the page we will actually inspect.
             */
            slotno = SimpleLruReadPage_ReadOnly(NotifyCtl, curpage,
                                                InvalidTransactionId);
            if (curpage == QUEUE_POS_PAGE(head))
            {
                /* we only want to read as far as head */
                copysize = QUEUE_POS_OFFSET(head) - curoffset;
                if (copysize < 0)
                    copysize = 0;   /* just for safety */
            }
            else
            {
                /* fetch all the rest of the page */
                copysize = QUEUE_PAGESIZE - curoffset;
            }
            memcpy(page_buffer.buf + curoffset,
                   NotifyCtl->shared->page_buffer[slotno] + curoffset,
                   copysize);
            /* Release lock that we got from SimpleLruReadPage_ReadOnly() */
            LWLockRelease(NotifySLRULock);
 
            /*
             * Process messages up to the stop position, end of page, or an
             * uncommitted message.
             *
             * Our stop position is what we found to be the head's position
             * when we entered this function. It might have changed already.
             * But if it has, we will receive (or have already received and
             * queued) another signal and come here again.
             *
             * We are not holding NotifyQueueLock here! The queue can only
             * extend beyond the head pointer (see above) and we leave our
             * backend's pointer where it is so nobody will truncate or
             * rewrite pages under us. Especially we don't want to hold a lock
             * while sending the notifications to the frontend.
             */
            reachedStop = asyncQueueProcessPageEntries(&pos, head,
                                                       page_buffer.buf,
                                                       snapshot);
        } while (!reachedStop);
    }
    PG_FINALLY();
    {
        /* Update shared state */
        LWLockAcquire(NotifyQueueLock, LW_SHARED);
        QUEUE_BACKEND_POS(MyBackendId) = pos;
        LWLockRelease(NotifyQueueLock);
    }
    PG_END_TRY();
 
    /* Done with snapshot */
    UnregisterSnapshot(snapshot);
}
 
/*
 * Fetch notifications from the shared queue, beginning at position current,
 * and deliver relevant ones to my frontend.
 *
 * The current page must have been fetched into page_buffer from shared
 * memory.  (We could access the page right in shared memory, but that
 * would imply holding the NotifySLRULock throughout this routine.)
 *
 * We stop if we reach the "stop" position, or reach a notification from an
 * uncommitted transaction, or reach the end of the page.
 *
 * The function returns true once we have reached the stop position or an
 * uncommitted notification, and false if we have finished with the page.
 * In other words: once it returns true there is no need to look further.
 * The QueuePosition *current is advanced past all processed messages.
 */
static bool
asyncQueueProcessPageEntries(volatile QueuePosition *current,
                             QueuePosition stop,
                             char *page_buffer,
                             Snapshot snapshot)
{
    bool        reachedStop = false;
    bool        reachedEndOfPage;
    AsyncQueueEntry *qe;
 
    do
    {
        QueuePosition thisentry = *current;
 
        if (QUEUE_POS_EQUAL(thisentry, stop))
            break;
 
        qe = (AsyncQueueEntry *) (page_buffer + QUEUE_POS_OFFSET(thisentry));
 
        /*
         * Advance *current over this message, possibly to the next page. As
         * noted in the comments for asyncQueueReadAllNotifications, we must
         * do this before possibly failing while processing the message.
         */
        reachedEndOfPage = asyncQueueAdvance(current, qe->length);
 
        /* Ignore messages destined for other databases */
        if (qe->dboid == MyDatabaseId)
        {
            if (XidInMVCCSnapshot(qe->xid, snapshot))
            {
                /*
                 * The source transaction is still in progress, so we can't
                 * process this message yet.  Break out of the loop, but first
                 * back up *current so we will reprocess the message next
                 * time.  (Note: it is unlikely but not impossible for
                 * TransactionIdDidCommit to fail, so we can't really avoid
                 * this advance-then-back-up behavior when dealing with an
                 * uncommitted message.)
                 *
                 * Note that we must test XidInMVCCSnapshot before we test
                 * TransactionIdDidCommit, else we might return a message from
                 * a transaction that is not yet visible to snapshots; compare
                 * the comments at the head of heapam_visibility.c.
                 *
                 * Also, while our own xact won't be listed in the snapshot,
                 * we need not check for TransactionIdIsCurrentTransactionId
                 * because our transaction cannot (yet) have queued any
                 * messages.
                 */
                *current = thisentry;
                reachedStop = true;
                break;
            }
            else if (TransactionIdDidCommit(qe->xid))
            {
                /* qe->data is the null-terminated channel name */
                char       *channel = qe->data;
 
                if (IsListeningOn(channel))
                {
                    /* payload follows channel name */
                    char       *payload = qe->data + strlen(channel) + 1;
 
                    NotifyMyFrontEnd(channel, payload, qe->srcPid);
                }
            }
            else
            {
                /*
                 * The source transaction aborted or crashed, so we just
                 * ignore its notifications.
                 */
            }
        }
 
        /* Loop back if we're not at end of page */
    } while (!reachedEndOfPage);
 
    if (QUEUE_POS_EQUAL(*current, stop))
        reachedStop = true;
 
    return reachedStop;
}
 
/*
 * Advance the shared queue tail variable to the minimum of all the
 * per-backend tail pointers.  Truncate pg_notify space if possible.
 *
 * This is (usually) called during CommitTransaction(), so it's important for
 * it to have very low probability of failure.
 */
static void
asyncQueueAdvanceTail(void)
{
    QueuePosition min;
    int         oldtailpage;
    int         newtailpage;
    int         boundary;
 
    /* Restrict task to one backend per cluster; see SimpleLruTruncate(). */
    LWLockAcquire(NotifyQueueTailLock, LW_EXCLUSIVE);
 
    /*
     * Compute the new tail.  Pre-v13, it's essential that QUEUE_TAIL be exact
     * (ie, exactly match at least one backend's queue position), so it must
     * be updated atomically with the actual computation.  Since v13, we could
     * get away with not doing it like that, but it seems prudent to keep it
     * so.
     *
     * Also, because incoming backends will scan forward from QUEUE_TAIL, that
     * must be advanced before we can truncate any data.  Thus, QUEUE_TAIL is
     * the logical tail, while QUEUE_STOP_PAGE is the physical tail, or oldest
     * un-truncated page.  When QUEUE_STOP_PAGE != QUEUE_POS_PAGE(QUEUE_TAIL),
     * there are pages we can truncate but haven't yet finished doing so.
     *
     * For concurrency's sake, we don't want to hold NotifyQueueLock while
     * performing SimpleLruTruncate.  This is OK because no backend will try
     * to access the pages we are in the midst of truncating.
     */
    LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
    min = QUEUE_HEAD;
    for (BackendId i = QUEUE_FIRST_LISTENER; i > 0; i = QUEUE_NEXT_LISTENER(i))
    {
        Assert(QUEUE_BACKEND_PID(i) != InvalidPid);
        min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
    }
    QUEUE_TAIL = min;
    oldtailpage = QUEUE_STOP_PAGE;
    LWLockRelease(NotifyQueueLock);
 
    /*
     * We can truncate something if the global tail advanced across an SLRU
     * segment boundary.
     *
     * XXX it might be better to truncate only once every several segments, to
     * reduce the number of directory scans.
     */
    newtailpage = QUEUE_POS_PAGE(min);
    boundary = newtailpage - (newtailpage % SLRU_PAGES_PER_SEGMENT);
    if (asyncQueuePagePrecedes(oldtailpage, boundary))
    {
        /*
         * SimpleLruTruncate() will ask for NotifySLRULock but will also
         * release the lock again.
         */
        SimpleLruTruncate(NotifyCtl, newtailpage);
 
        /*
         * Update QUEUE_STOP_PAGE.  This changes asyncQueueIsFull()'s verdict
         * for the segment immediately prior to the old tail, allowing fresh
         * data into that segment.
         */
        LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
        QUEUE_STOP_PAGE = newtailpage;
        LWLockRelease(NotifyQueueLock);
    }
 
    LWLockRelease(NotifyQueueTailLock);
}
 
/*
 * ProcessIncomingNotify
 *
 *      Scan the queue for arriving notifications and report them to the front
 *      end.  The notifications might be from other sessions, or our own;
 *      there's no need to distinguish here.
 *
 *      If "flush" is true, force any frontend messages out immediately.
 *
 *      NOTE: since we are outside any transaction, we must create our own.
 */
static void
ProcessIncomingNotify(bool flush)
{
    /* We *must* reset the flag */
    notifyInterruptPending = false;
 
    /* Do nothing else if we aren't actively listening */
    if (listenChannels == NIL)
        return;
 
    if (Trace_notify)
        elog(DEBUG1, "ProcessIncomingNotify");
 
    set_ps_display("notify interrupt");
 
    /*
     * We must run asyncQueueReadAllNotifications inside a transaction, else
     * bad things happen if it gets an error.
     */
    StartTransactionCommand();
 
    asyncQueueReadAllNotifications();
 
    CommitTransactionCommand();
 
    /*
     * If this isn't an end-of-command case, we must flush the notify messages
     * to ensure frontend gets them promptly.
     */
    if (flush)
        pq_flush();
 
    set_ps_display("idle");
 
    if (Trace_notify)
        elog(DEBUG1, "ProcessIncomingNotify: done");
}
 
/*
 * Send NOTIFY message to my front end.
 */
void
NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
{
    if (whereToSendOutput == DestRemote)
    {
        StringInfoData buf;
 
        pq_beginmessage(&buf, 'A');
        pq_sendint32(&buf, srcPid);
        pq_sendstring(&buf, channel);
        pq_sendstring(&buf, payload);
        pq_endmessage(&buf);
 
        /*
         * NOTE: we do not do pq_flush() here.  Some level of caller will
         * handle it later, allowing this message to be combined into a packet
         * with other ones.
         */
    }
    else
        elog(INFO, "NOTIFY for \"%s\" payload \"%s\"", channel, payload);
}
 
/* Does pendingNotifies include a match for the given event? */
static bool
AsyncExistsPendingNotify(Notification *n)
{
    if (pendingNotifies == NULL)
        return false;
 
    if (pendingNotifies->hashtab != NULL)
    {
        /* Use the hash table to probe for a match */
        if (hash_search(pendingNotifies->hashtab,
                        &n,
                        HASH_FIND,
                        NULL))
            return true;
    }
    else
    {
        /* Must scan the event list */
        ListCell   *l;
 
        foreach(l, pendingNotifies->events)
        {
            Notification *oldn = (Notification *) lfirst(l);
 
            if (n->channel_len == oldn->channel_len &&
                n->payload_len == oldn->payload_len &&
                memcmp(n->data, oldn->data,
                       n->channel_len + n->payload_len + 2) == 0)
                return true;
        }
    }
 
    return false;
}
 
/*
 * Add a notification event to a pre-existing pendingNotifies list.
 *
 * Because pendingNotifies->events is already nonempty, this works
 * correctly no matter what CurrentMemoryContext is.
 */
static void
AddEventToPendingNotifies(Notification *n)
{
    Assert(pendingNotifies->events != NIL);
 
    /* Create the hash table if it's time to */
    if (list_length(pendingNotifies->events) >= MIN_HASHABLE_NOTIFIES &&
        pendingNotifies->hashtab == NULL)
    {
        HASHCTL     hash_ctl;
        ListCell   *l;
 
        /* Create the hash table */
        hash_ctl.keysize = sizeof(Notification *);
        hash_ctl.entrysize = sizeof(NotificationHash);
        hash_ctl.hash = notification_hash;
        hash_ctl.match = notification_match;
        hash_ctl.hcxt = CurTransactionContext;
        pendingNotifies->hashtab =
            hash_create("Pending Notifies",
                        256L,
                        &hash_ctl,
                        HASH_ELEM | HASH_FUNCTION | HASH_COMPARE | HASH_CONTEXT);
 
        /* Insert all the already-existing events */
        foreach(l, pendingNotifies->events)
        {
            Notification *oldn = (Notification *) lfirst(l);
            NotificationHash *hentry;
            bool        found;
 
            hentry = (NotificationHash *) hash_search(pendingNotifies->hashtab,
                                                      &oldn,
                                                      HASH_ENTER,
                                                      &found);
            Assert(!found);
            hentry->event = oldn;
        }
    }
 
    /* Add new event to the list, in order */
    pendingNotifies->events = lappend(pendingNotifies->events, n);
 
    /* Add event to the hash table if needed */
    if (pendingNotifies->hashtab != NULL)
    {
        NotificationHash *hentry;
        bool        found;
 
        hentry = (NotificationHash *) hash_search(pendingNotifies->hashtab,
                                                  &n,
                                                  HASH_ENTER,
                                                  &found);
        Assert(!found);
        hentry->event = n;
    }
}
 
/*
 * notification_hash: hash function for notification hash table
 *
 * The hash "keys" are pointers to Notification structs.
 */
static uint32
notification_hash(const void *key, Size keysize)
{
    const Notification *k = *(const Notification *const *) key;
 
    Assert(keysize == sizeof(Notification *));
    /* We don't bother to include the payload's trailing null in the hash */
    return DatumGetUInt32(hash_any((const unsigned char *) k->data,
                                   k->channel_len + k->payload_len + 1));
}
 
/*
 * notification_match: match function to use with notification_hash
 */
static int
notification_match(const void *key1, const void *key2, Size keysize)
{
    const Notification *k1 = *(const Notification *const *) key1;
    const Notification *k2 = *(const Notification *const *) key2;
 
    Assert(keysize == sizeof(Notification *));
    if (k1->channel_len == k2->channel_len &&
        k1->payload_len == k2->payload_len &&
        memcmp(k1->data, k2->data,
               k1->channel_len + k1->payload_len + 2) == 0)
        return 0;               /* equal */
    return 1;                   /* not equal */
}
 
/* Clear the pendingActions and pendingNotifies lists. */
static void
ClearPendingActionsAndNotifies(void)
{
    /*
     * Everything's allocated in either TopTransactionContext or the context
     * for the subtransaction to which it corresponds.  So, there's nothing to
     * do here except reset the pointers; the space will be reclaimed when the
     * contexts are deleted.
     */
    pendingActions = NULL;
    pendingNotifies = NULL;
}